Most high current ion implantation systems rely on electron trapping to compensate for the space charge of the ion beam. Electrons generated by ion beam ionization of a background gas can be trapped when such ionization occurs within the positive potential of the beam. The rate of ionization is determined by the collision frequency between ions and gas molecules, N.sigma.1I.sub.b. N is the residual gas density, .sigma. is the cross section for the reaction, 1 is the beam path length, and I.sub.b is the ion beam current. At 20.degree. C., N is 3.3.times.10.sup.16 molecules.torr.sup.-1 cm.sup.-3. A typical cross section .sigma. is 10.sup.-15 cm.sup.2 and 1 is for example, 20 cm within a high gas pressure region of a typical implanter. Typical implant pressures are 10.sup.-5 to 10.sup.-4 torr which leads to an ionization rate of 0.0066 to 0.066 per beam particle. This ionization rate is adequate to maintain space charge neutrality if the electrons produced by the collisions are not lost due to external voltage sources attracting them from the beam.
In some high current ion implantation systems, a gas is deliberately leaked into the beam line to enhance the ionization process thereby providing space charge neutralization of the ion beam. The ionization efficiency of the ion beam at allowable gas pressures is, however, too low to permit use of a gas leak for control of wafer charging.
A plasma bridge can be used for neutralizing an ion beam and in principle can be used to control wafer surface charging simultaneously. Using this approach, a plasma is generated and bridged with the ion beam to provide a high current source of low energy electrons. The major shortcoming of this system is the high gas load presented to the system.
There are a number of known variations on the theme of electron shower systems. Most of these systems use low energy secondary electrons which are flooded directly onto wafers during implantation. The major shortcoming of this technique is that the spatial distribution of unlike charges is not equal. There is a negative halo around the positively charged beam center. This characteristic causes devices on the surfaces of wafers to be charged first negatively, then positively, and finally negatively, as the wafer passes through the beam. The net residual charge after the pass may be zero, but damage can be caused during the pass.
U.S. Pat. No. 4,463,255 to Robertson et al. recognizes disadvantages in direct electron shower charge neutralization of an implantation wafer. The U.S. Pat. No. 4,463,255 discloses a beam neutralization approach wherein a source of primary high energy electrons are directed to a target adjacent the ion beam. Secondary electrons produced as the primary electrons impact the target have low energy and can be trapped by the positively charged ion beam. This tends to neutralize he net charge of the beam and therefore reduces charge build up on the wafer as the ion beam impinges upon a wafer surface.